1. Technical Field
This invention relates generally to printed circuit board assembly, and more specifically, to a method and apparatus for facilitating the calibration of pick and place equipment.
2. Background of the Invention
Integrated and discrete components are commonly assembled onto printed circuit boards (PCBs) using pick and place equipment to set the various component parts on the board prior to attachment. Fuji America Corporation of Lincolnshire, Ill., as well as other manufacturers provide a variety of pick and place equipment.
The pick and place device typically includes a conveyor section which receives and transports a PCB onto an assembly table. The table is capable of movement in commonly two and occasionally in three axes. The PCB is positioned generally below at least one placing head.
Each placing head includes a part placement nozzle, which picks a component part typically employing a vacuum applied to the upper face of the component part. The component part is picked from a first position, for example from a tape-and-reel which dispenses a particular component part at a preselected position. After the part placement nozzle has picked the component part, the placing head moves laterally as required to position the component part over the PCB. The assembly table is also advanced along its X and Y axes as required to position the PCB for placement of the component part. As positioning occurs, the component part is also aligned with the PCB, by rotation of the placing head to orient the component part as required relative to the PCB.
The device next places the component part against the upper surface of the PCB in its preselected location, the part placement nozzle typically exerting pressure against the upper face of the component part, securing the component part against the face of the PCB. In a subsequent process step, the component part is permanently affixed to the surface of the PCB, often by heating a solder which has been previously applied to the surface of the PCB.
To ensure the proper alignment of the component part on the PCB, the pick and place equipment must be calibrated occasionally. Calibration may be achieved with actual electronic component parts and printed circuit boards. Using this method, the component parts are assembled onto the PCB by the pick and place equipment. The alignment of the device is checked using visual inspection with an unaided eye or using a microscope. If the component parts are determined to be misaligned in the X-, Y-, or .theta. (rotational) axes, adjustments to the pick and place equipment are required. The process of adjustment and visual inspection continues until an acceptable alignment of the component part with the PCB is achieved. A similar calibration system is described in U.S. Pat. No. 4,776,088.
Alternately, calibration may be achieved by the method and employing the apparatus disclosed in U.S. Pat. No. 5,537,204. The method and apparatus disclosed in U.S. Pat. No. 5,537,204 include loading a glass fiducial plate onto an assembly table of a pick and place machine, loading a glass chip onto a part placement nozzle of the pick and place machine, and placing the glass chip onto the plate. Alignment of a fiducial point on the fiducial plate with a target window on the chip is examined. Responsive to the alignment of the target and the fiducial, key calibration codes in calibration software of the pick and place machine may be adjusted. Once calibration is complete, multiple placement cycles using the glass fiducial plate and glass chips are performed to determine the statistical placement capability of the pick-and-plate equipment.
A problem associated with the method and apparatus described above involves the loading of the glass chip onto the part placement nozzle of the pick and place machine. In order for the glass chip to be placed onto the part placement nozzle the machines safety doors must be overridden by employing typically a key or set of keys which are inserted into a slot or slots, thereby disabling the safety doors. Once disabled, an operator manually places the glass chip on the end of the part placement nozzle by pressing a key temporarily stop movement of the machine then reaching into the device exposing the operator's arms and hands to the possibility of injury within the device. Additionally, because the glass chip is placed manually on the end of the nozzle, it is difficult for the operator to assure proper alignment of the glass chip. While the pick and place machine has the capability of determining the extent to which a component part has been picked askew of a desired rotational orientation, the rapid movement of the nozzle against the surface of the glass chip may create a situation wherein the part continues to rotate even though the nozzle has ceased rotation due to the relatively low coefficient of friction of the surface of the glass chip.
What is needed is a method and apparatus which allows for the safe and accurate pick-up of glass chips by the part placement nozzle of the pick and place equipment during calibration.